Typical FM-CW radar apparatuses using millimeter waves used for, for example, preventing collisions of automobiles (hereinafter, simply referred to as “radar apparatuses”) emit transmitted waves whose frequencies are changed in a time-varying manner, receive reflected waves generated by reflection of the transmitted waves from target objects, and calculate relative distances between automobiles equipped with the radar apparatuses and the target objects and relative velocities of the target objects with respect to the automobiles using the transmitted waves and the received waves.
In such radar apparatuses, how accurately the relative distances and the relative velocities can be calculated is recognized as a major challenge. In order to solve this, various radar apparatuses have been disclosed (for example, see Patent Documents 1 and 2).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-326504
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-98753
Known radar apparatuses including those described in the above-described patent documents generate beat signals using transmitted waves and received waves, and detect the frequency spectra of the beat signals. When transmitted waves having a triangular waveform are used, for example, peaks in the frequency spectra of beat signals in frequency up-modulation sections and those in frequency down-modulation sections are correspondingly paired and used for calculating relative distances or relative velocities.
However, as shown in FIGS. 10(A) to 10(C), correct pairing can be difficult when a plurality of received waves exist for one transmitted wave.
FIG. 10(A) illustrates waveforms of a transmitted wave and received waves in the case where a plurality of received waves resulting from one transmitted wave are measured, FIG. 10(B) illustrates a frequency spectrum in an up-modulation section, and FIG. 10(C) illustrates a frequency spectrum in a down-modulation section.
More specifically, when received waves RX1 and RX2 resulting from a transmitted wave TX and delayed from the transmitted wave by delay times τ1 and τ2, respectively, are measured, a peak a1u corresponding to a frequency difference f1a between the frequency of the transmitted wave TX and that of the received wave RX1 and a peak b1u corresponding to a frequency difference f1b between the frequency of the transmitted wave TX and that of the received wave RX2 are generated so as to be close to each other in an up-modulation section. On the other hand, a peak a1d corresponding to a frequency difference f1c between the frequency of the transmitted wave TX and that of the received wave RX1 and a peak b1d corresponding to a frequency difference f1d between the frequency of the transmitted wave TX and that of the received wave RX2 are generated so as to be close to each other in a down-modulation section. At this moment, relative velocities and relative distances cannot be calculated accurately unless the spectral peaks in the up-modulation section and those in the down-modulation section are combined (paired) such that the original signals (transmitted wave and received waves) are the same.
To date, spectral peaks having the same amplitudes of frequency components or the same amounts of frequency change have been paired when a plurality of received waves resulting from one transmitted wave are measured as described above. However, correct pairing is difficult by such a method when the amplitudes or the amounts of frequency change are continuously constant as in, for example, waves reflected from median strips or guard rails on roads. Thus, relative velocities or relative distances cannot be accurately calculated.